(CNN) -- Authorities investigating the case of a boy who disappeared in Kansas almost a decade ago plan to search an undisclosed residence Wednesday, the Butler County sheriff said.

An age-progression photo shows what Adam Herrman would like today, as a 21-year-old man.

Sheriff Craig Murphy would not disclose details about the residence or why authorities want to search it.

He said his department will also search on an area of the Whitewater River, in southern Kansas, on Saturday near where Adam Herrman was last seen.

Adam was 11 when he went missing in 1999. He was living in a mobile home park in Towanda, a town about 25 miles northeast of Wichita, with his adoptive parents, Doug and Valerie Herrman, authorities said.

Wichita attorney Warner Eisenbise, who is representing Adam's adoptive parents, said the couple believed Adam had run away and didn't report him missing. They "really rue the fact that they didn't" report him missing, he said Monday.

A few weeks ago, an undisclosed person contacted the Wichita-Sedgwick County Exploited and Missing Child Unit, expressing concern about Adam, the sheriff said.

The Herrmans told Eisenbise that Adam ran away frequently, the attorney said, and they believed he was either with his biological parents or homeless. Although the Herrmans did not report him missing, "they were very worried about him," Eisenbise said.

Don't Miss

In an interview published Tuesday in The Wichita Eagle, Valerie Herrman said Adam ran away in May 1999 after she spanked him with a belt. She said she was upset but doesn't remember why, The Eagle reported.

The couple never reported Adam missing, Valerie Herrman told the paper, because they feared authorities would take Adam and his siblings away because of the spanking.

The couple adopted his two younger siblings as well, according to The Eagle.

"We love him, and we made a terrible mistake" by not reporting him missing, Doug Herrman told The Eagle. The couple said they searched the mobile home park and other areas for two days after Adam left.

"Then we came to the conclusion that the police probably have him, and they're coming to us, probably to get us in trouble," Doug Herrman told the newspaper, but the "police never came."

Authorities have searched an empty lot in the Pine Ridge Mobile Home Park where the family lived. There, police found an "answer" to one of their questions, Murphy said Monday without elaborating.

Eisenbise said that on December 15, authorities also searched the Herrmans' homes in Derby, outside of Wichita, and took the couple's computer, he said.

The National Center for Missing and Exploited Children has released an age-progression picture that depicts Adam as he might appear now: a young man with blue eyes and light-colored hair.

Adam had been placed in the Herrmans' care when he was about 2, Murphy said Monday.

He had been named Irvin Groeninger III when he was born June 8, 1987, Murphy said, and it was not clear when his name was changed. His biological parents relinquished their rights as parents about two decades ago, and Adam and his siblings were put in foster homes, CNN affiliate KWCH reported.

"I thought what I was doing for them was in the best interest of the children, and evidently it wasn't," Irvin Groeninger, Adam's biological father, told KWCH. "If he was still in my custody, this would have never happened."

Adam's sister, Tiffany Broadfoot, 22, said she had last seen her brother about 14 years ago at a birthday party.

"He had the cutest little round face, little-bitty freckles right up here on the tip of his cheek," she told the station.